Comparisons of ambulatory blood pressure monitoring and repeated office measurements in primary care

BACKGROUND. The accuracy of office blood pressure (BP) readings is questionable because of blood pressure variability and measurement errors. The primary aim of this study was to determine the number of office visits required to optimize the estimation of usual blood pressure in older adults in primary care. METHODS. Ambulatory blood pressure monitoring was used to define usual blood pressure in an observational study of 75 randomly selected family practice patients. Each subject made six visits for office BP measurements and had 24-hour ambulatory BP monitoring done twice. Mean office BP, based on one through six visits, was compared with mean ambulatory BP. RESULTS. The sample consisted of 29 men and 46 women; 18 were black and 57 were white. Twenty-one subjects were taking antihypertensive medication. The mean age ± 1 /standard deviation (SD) was 60 (±8) years. The correlation between mean office BP and mean ambulatory BP rose with the number of visits averaged, with most of the gain obtained within 3 visits. The maximal correlation for 24-hour ambulatory BP was r = .85/.75 (systolic/diastolic) (P <.01). However, even when using average office BP over six visits to estimate mean ambulatory BP, a discrepancy of ≤10 mm Hg between estimated and observed ambulatory BP levels persisted in 18% to 20% of subjects. CONCLUSIONS. Readings from at least three office visits should be averaged to estimate usual blood pressure. It should be noted, however, that important discrepancies between estimated and observed mean ambulatory BP persist even after readings taken over six visits. Ambulatory BP monitoring probably provides unique information about usual blood pressure that cannot be captured by repeated office BP readings.